For a number of years, tire manufacturers have been devoting significant effort to developing novel solutions to a problem that dates back to the very start of the use of wheels fitted with tires of the inflated type, i.e. how to allow the vehicle to continue on its journey in spite of a substantial or complete loss of pressure from one or more tires. For decades, the spare wheel was considered to be the only and universal solution.
Then, more recently, the considerable advantages associated with omitting the spare wheel appeared. The idea of “extended mobility” was developed. The associated techniques allow a vehicle to continue to drive along on the same tire, even after a puncture or significant loss of pressure, provided that certain instructions are observed. This, for example, means that a service area can be reached without the need to stop, under circumstances which are often dangerous, in order to fit the spare wheel.
Today, there are two major types of run-flat technology. On the one hand, there are wheels fitted with supports capable of supporting the inside of the tread of a tire when the sidewalls collapse following a loss of pressure. This solution is advantageously combined with a tire that has a bottom region capable of minimizing the risks of the tire slipping off the rim. This solution has the advantage that the tire performance under normal conditions is very close to the performance of a conventional tire. By contrast, it does have the disadvantage of entailing the use of an additional component, namely the support, for each of the wheels of the vehicle.
On the other hand, there are tires of the self-supporting type (often known by their English-language abbreviation “ZP” which stands for “zero pressure”). These self-supporting tires are able to support a significant load at a reduced pressure, or even with no pressure, because they have sidewalls which are reinforced, usually by means of rubber inserts provided in the sidewalls. For the sake of simplicity, these tires will hereinafter be referred to as “run-flat tires”.
Tires such as this are known, for example, from U.S. Pat. Nos. 4,067,347, 4,779,658, 5,511,599, 5,769,980, 5,795,416, 6,022,434 and 7,093,633.
The most widespread design of run-flat tires involves anchoring the carcass reinforcement by wrapping it around the annular reinforcing structure of the bead, so as to form, within each bead, an incoming portion extending from the crown through the sidewalls towards the bead and a wrapped-around portion, the radially outer end of the wrapped-around portion being radially close to the “equator” of the tire, that is to say radially close to the radial height at which the carcass reinforcement, when the tire is mounted on the rim, is at its largest axial width. The space formed between the incoming portion and the wrapped-around portion of the carcass reinforcement is filled with a layer of rubber mix known as the apex rubber, as is depicted in FIG. 1.
These tires, particularly when greatly flexed may, both in “normal running mode” (that is to say when inflated to a pressure close to their service pressure) and in “run-flat mode” (that is to say when the inflation pressure of the tire is significantly reduced with respect to the service pressure, or even when there is no inflation pressure at all), give rise to endurance problems in the “bottom region” that is to say in the beads and in the radially inner part of the sidewalls, the cause for this generally being attributed to the fact that the wrapped-around portion of the carcass reinforcement may find itself under compression when the bead, which is bearing against the rim hook, bends about the hook (as depicted in FIG. 10). As a matter of fact, because of the great thickness of the rubber mix between the incoming portion and the wrapped-around portion of the carcass reinforcement, the wrapped-around portion may find itself under significant compressive stress resulting from the bending of the radially inner part of the sidewall, (which can be likened to a beam) about the rim hook.
In order to address this problem, it has been proposed that the thickness of the rubber mix between the incoming portion and the wrapped-around portion of the carcass reinforcement be reduced significantly (giving rise to a tire “with close-fitting wrapped-around portion”) and/or to reduce significantly the radial height of the wrapped-around portion (this giving a “tire with shortened wrapped-around portion”). While a wrapped-around portion configuration such as this has proven advantageous from a number of standpoints (better endurance in “normal running mode”, reduction in the length of the carcass reinforcement, and therefore in the weight and cost of manufacture, option to use reinforcing elements with lower compressive strength, etc.), it has not yielded entirely satisfactory results particularly under run-flat conditions.